The present invention is related to the treatment of cellulosic material, such as wood pulp, and, more particularly, to a continuous treatment digester of the moving conveyor type.
It is well known in the art to chemically treat cellulosic materials to produce cellulosic pulp by impregnating the cellulosic material with a treatment liquid that dissolves lignin and digests the impregnated material at elevated temperature and pressure by inducing a solvent action on lignin to free cellulosic fibers. One such process involves the utilization of a single digester vessel for both chemical impregnation and vapor phase digesting or cooking of the impregnated material. Such a process, as well as an apparatus for carrying out the process, is disclosed in U.S. Pat. No. 3,549,483.
As disclosed therein, cellulosic material is continuously introduced into a warm cooking liquor housed in the lower portion of an inclined elongated cylindrical vessel. The incoming material is deposited onto a conveyor which is disposed along the axis of the vessel and serves to carry the material through the cooking liquor and then into the cooking vapor housed within the vessel above the liquid-vapor interface. The material may be passed either concurrently or countercurrently through the cooking liquor, thence through the vapor liquor interface and into the vapor phase to an outlet located within the vapor phase enclosing portion of the inclined vessel. The digested material withdrawn from the vessel may undergo further processing downstream thereof as is well known in the art, such as cooking, washing, bleaching, refining and the like.
Continuous feeding of cellulosic material to be treated within the vessel has always been a problem. The most conventional technique for feeding fibrous material to the vessel has been to meter uncompacted chips in the presence of air into a rotary valve or screw feeder which convey the chips to a gas filled chamber which opens through the top side of the inclined vessel. The chips passing from the rotary valve or the screw feeder then fall by gravity through the gas filled chamber into the pockets between the flights of the conveyor and are then picked up by the flights of the conveyor and conveyed into and through the treatment liquid. However, it is difficult to obtain uniform feed between flights of the conveyor and to fill the volume between flights across the entire width of the conveyor. Feeding techniques of this type are disclosed in U.S. Pat. Nos. 3,111,832 and 3,135,651.
Another technique for feeding cellulosic material to a digester vessel is disclosed in U.S. Pat. No. 3,549,483 wherein a screw feeder is used to feed cellulosic material through an opening located in the underside of the inclined vessel at a point below the liquid level within the vessel. Again, the material is fed through a single opening in the vessel so as to fall between the flights of the conveyor as the material discharges from the screw feeder and to be picked up by the flights and conveyed through the treatment liquid. This technique also suffers from the inability to uniformly fill the volume of the pockets between flights with material across the entire width of the conveyor.